At present allergy, manifested as atopic diseases—atopic eczema, allergic rhinitis and asthma—represents a chronic disorder of rising importance in economically developed countries world-wide. The demonstration of an inverse association between infections early in life and atopy represents a substantial advance which has led to renewed scientific interest in the hygiene hypothesis introduced a decade ago, according to which the recent rapid increase in atopy may in fact be due to improved hygiene and reduced family size. Recent epidemiological studies have yielded results both for and against this hypothesis.
Gastrointestinal microflora promote processes with a potential to counter allergy:
1) T helper 1-type immunity, 2) generation of transforming growth factor-β (TGF-β), which has a vital role both in the suppression of Th2-induced allergic inflammation and in induction of oral tolerance and 3) IgA production, an indispensable component in the mucosal immune defence (Sanfilippo et al., 2000; Isolauri et al., 2000). The gut microflora may thus represent a major postnatal counter-regulator of the universal Th2-skewed immune system of pregnancy and neonatal age. Confrontation with microbial antigens in the gastrointestinal tract begins instantly after birth, and the viable cells of a fully established gut microflora outnumber those of the host by a factor of ten. Consequently, commensal gastrointestinal microbes constitute the earliest and most substantial stimulus for the development of gut-associated lymphoid tissue.
Probiotics, defined as live microbial food ingredients beneficial to health, are normal commensal bacteria of the healthy human gut microflora. The most frequently used genera are lactobacilli and bifidobacteria. Probiotics are known to reverse the increased intestinal permeability characteristic of children with atopic eczema and food allergy, and to enhance gut-specific IgA responses, frequently defective in children with food allergy (Isolauri et al., 1993; Majamaa and Isolauri, 1996; Isolauri, 1992). Promotion of gut barrier function by means of probiotics also includes normalization of the gut microecology, alterations in which have been demonstrated in allergic subjects. Recent studies indicate that certain probiotics alleviate changes related to allergic inflammation both in vitro and in vivo (Isolauri et al., 2000; Majamaa and Isolauri, 1997). A probiotic strain, Lactobacillus casei, ssp. rhamnosus (Lactobacillus GG; ATCC 53103), has been proven safe at an early age and effective in the treatment of allergic inflammation and food allergy. Specific strains in the gut indigenous microflora thus exert profound effects on the physiology and immunology of the host.
At birth the human gastrointestinal tract is sterile, but during the first months and years of life a rapid sequential colonization occurs, leading to stable indigenous gut microflora. Simultaneously, the universally Th2-dominant immunity of newborns is intensified in atopic individuals, with the subsequent expression of atopic disease. In addition to normal microflora impacting on the neonatal gastrointestinal system, another massive and constant antigen source is confronted in the form of dietary antigens. Recent animal studies indicate that these may potentiate atopic-type immunity at both mucosal and systemic level. Any process designed to counter-regulate allergy must thus exert its major effects in infancy, and preferably during the initial encounters with dietary antigens. Here probiotics are eminently appropriate for the task, not only as regards timing but also in their capacity to reduce the dietary antigen load by degrading and modifying macromolecules (Pessi et al., 1998).